Cathode structure for magnetrons



May 8, 1951 v I R. L. JEPsl-:N 2,552,045

CATHODE STRUCTURE FOR MAGNE'IRONS Filed sept. 11, 1946 4 4 4j l da 6:2 72 fz fa 4l 46 f2 IN VEN TOR.

TRjl/EK Patented May 8, 1951 Robert L. Jepsen, Lancaster, Pa., assignor to Radio Corporation of America, a corporation of Delaware Application September 11, 1946, Serial No. 696,152

Claims. 1 Y

The present invention relates to electron discharge tubes, and more especially to discharge tubes of the magnetrontype.

In the development of a particular tunable continuous wave magnetron, it was found desirable to provide a cathode which could provide current densities of from l'vto 3 amperes per square centimeter. The use of a conventional oxide coated type of cathode providing such large current densities has thus far proved impractical. .Another vital consideration in these tubes of the magnetron type has been the problem of back bombardment of the cathode. This back bombardment is caused by the return of electrons to thecathode due lto conditions existing during tube operation. The returning electrons arrive at the cathode with excess energy picked up in the interelectrode space. The excess energy of the electrons couldbe derived, for example, by a transfer of energyv from the source of alternating plate voltage to those electrons leaving the cathode at the wrong time in the cycle of alternating voltage. In an electron tube of this type in which, for example, it is desirable Vto have an output of 100 watts, the back bombardment power can amount to something like 5 to 15 watts. For high frequency magnetrons, this would be in excess of the normal power required to heat an ordinary oxide-coated cathode of thermally efficient design. In addition, the factors influencing tube operation may temporarily act in a fashion such that back bombardment is greatly increased. If the cathode normally derives the major portion of its power from the back bombardment (which an oxide coated cathode usually does), then increasing the back Ybombardment by an appreciable factor can result in destructivevaporization (due to increased temperature) of the oxide coating, and, perhaps, of the base metal as well. In some cases, means have beenprovided to control either the cathode heating current or the plate voltage for partially eliminating this destructive effect of cathode back bombardment. Other means have 'been provided for eliminating overheating of the cathode by liquid cooling. However, such methods complicate tube structure and do not function in an entirely satisfactory manner.

It is an object of my invention, therefore, to provide a cathode structure which will give a cathode current density of the order of amperes per square centimeter. It is a further object of my invention to provide a cathode structure which will effectively dissipate the excess energy of severe electron back bombardment.

'I'he novel features which I believe to be char- (Cl. Z50-27.5)

acteristic of my inventionare set forth with particu-larity in the appended claims, but the invention itself will best be understood by reference to the following description taken in connection with the accompanying drawing, in which:

Fig. 1 is `a partial longitudinal cross-sectional view of a` discharge tube according to my invention; and

Fig. 2 Yis a partial longitudinal cross-sectional view of a discharge tube according to a modification of my invention.

In some tubes of the magnetron type the -electrons from a large part of the cathode-anode dischargeacquire anincrease in their kinetic energy from collisions with other electrons'or 'from the changing electrostatic fields within the interelectrode space. As pointed out above these electrons circle back and strkethe cathode with a greaterkinetic energy than vpossessed Aby the electrons leavingv the Same electrode. This electron bombardment usually causes oxide coated cathode structures of conventional design to assume destructively high temperatures during operation. Excessive operating temperatures for oxide cathddeS Call 4IlOlr'nllly be avoided 'only by rather complex, and often unsatisfactory, expediencies. A thermally inefficient cathode is necessary for the safe dissipation ofboth constant and fluctuating back bombardment power. 'It is my desire inY this particular construction toaccomplish the necessary thermal inefficiency by utilizing a cathode `structure which operates at a high temperature. Since power radiated is proportional tothe fourth power of the absolute temperature, .an increase `in cathode temperature by a factor of 2 increases the radiated power by a vfactor of 16. Thermal ineiiicency in an oxide cathode must be achievedA by what are often clumsy vconduction methods. In the case ofa high temperature cathode,..the required thermal inefficiency oftenexists inherently because of the power that is radiatedfrcm the cathode surface.

Figure 1 discloses an electron discharge tube .of the magnetron type having anenvelope II. Mountedsofas to extend within the envelopeis a cathode electrode structure I3 having an electron emitting -cathode cylinder I0. spaced radially from cylinder IIJ is an annularly shapedanode I 5 which isconcentric with the axis `of the cylinder I0. .Extending radially and inwardly from the annulus ofthe `anode I5 arens I9 which form between themselves cavity resonators as is Well known-in the art. Spaced radially from the anode ring. I5 and mounted outside the envelope I I is an electromagnetic coil V23 for providing an axial vder 20-2I and an inner support rod I8.

magnetic field within the interelectrode space of the tube.

Other means may be provided for producing a magnetic eld between the anode ring and the cathode emitter. As is shown in Fig. 2, a permanent magnet having steel pole pieces 5B and 52 may be used to provide an axial magnetic neld in the interelectrode space between anode 42 and a cathode emitter 40 of a tube as is Well known in the art.

The electron emitting cylinder I0 in Fig. 1 is made preferably of a thin tantalum sheet. Mounted within the cylinder I0 is a tungsten heater rod I2. The heater rod I2 is joined to the tantalum cylinder I0 at one end by means of a sleeve I4 slipped over the end of the tungsten rod I2. Sleeve I4 can be made of tungsten, tantalum or molybdenum as is desired. Tungsten rod I2 and electron emitting cylinder Ill are supported by a coaxial lead structure I6 sealed through the envelope II. This coaxial support structure comprises a two part cylin- The 4two part supporting cylinder 20-2I may be made, if desired, of a single piece. Cylinder 2U is preferably of Kovar. Fitted to the upper end of cylinder 20 is the cylindrically shaped part 2| made preferably of molybdenum. Part 2| has a neck portion Which supports the tantalum cylinder IIJ by close mounting. Cylinder 2% can be joined to part 2|, preferably, by brazing. Support rod I8 is mounted within the cylinder 20 by aglass seal 22 at one end thereof. The upper end of supporting rod I8 is drilled with'a hole into which is mounted the tungsten heater rod I2. Heating current is supplied to this cathode structure by electrical leads 30 and 32, which are respectively connected to the center support rod I8 and the outer cylinder 20. Both the supporting rod I8 and the supporting cylinder structure 20-2I form electrical conductors for supplying a heating current to the tantalum cylinder I0 and heater rod I2. Cathode hats 24 and 26 are provided to confine the electron discharge between the anode fins I9 and the emitting surface of cylinder I0. These hats 24 and 26 are enlarged somewhat to vprovide an increased radiating area. They are formed of either tantalum, tungsten, or molybdenum. To keep the electron emission low, tungsten would be best; but machining diiculties dictate that either tantalum or molybdenum should be used. As the tantalum cylinder Ill will be raised to a high temperature during normal tube operation, it is desirable to prevent the molybdenum mounting part 2| from electron emission. I have provided a thin sleeve of tantalum 23 which can be slipped over the end of the mounting part 2|. This sleeve 28 will reduce electron emission from the outer molybdenum support 2 I.

The assembling of this cathode structure is comparatively simple. It is not necessary to braze or weld the various parts together. The center conductor rod I8 can be easily placed into vthe outer conductor cylinder 20 with a centering jig in the cathode end to maintain concentricity. The molybdenum end spacer I4 may be constructed slightly smaller in inner diameter than that of the tungsten rod I2. The rod I2 is tapered at the upper end by electropolishing and the end spacer I4 forced on this end of the rod as far as it will go. The tungsten rod with the molybdenum end spacer is then cut to the desired length. The hole I1 in the end of fill the support rod I8 may be made slightly smaller than the tungsten rod I2. The lower end of the tungsten rod is tapered by electropolishing, for example, and the rod I2 mounted tightly into the hole I1. The tantalum cylinder Ill is of thin metal. If the cylinder I0 can be rolled from a sheet it Will have a small slit along its length. This permits slight deformations which will allow it to be tted snugly into the outer supporting conductor 2|. The tantalum sleeve 28 is made of a size to be slipped over the upper end of the conductor 2|. The hats 24 and 26 can be tted onto cylinder I0. The use of tantalum makes possible a squeezing of the hats slightly to reduce the inside diameter in case they are a bit large. It is thus apparent that the parts can be put together in a mechanically snug fashion. Then, when the current ilows through the cathode, electrode junctions of high electrical resistance become hot and an actual sintering takes place. After a few hours of 0peration, I have found that most of the parts will be held as securely as though they had been welded. It is not necessary to make the cathode of so many parts. If desired, many of the parts can be combined in one piece machined to the proper configuration. For example, in Fig. 2 there is shown a cathode structure in which a single cathode element 40 conforms with portions 2|, IO, I4, 24, and 26 of Fig. 1. That is, this portion 40 may be machined from one piece and brazed to a Kovar cylinder 64. Furthermore, a tungsten rod 68 similar to rod I2 of Fig. 1 may Ibe mounted in the other end of portion 40. Then rod 68 may be mounted in a conductor rod 66 in a manner similar to that disclosed in Fig. 1 and for the same purpose.

The electron emitting cylinder I0 of Fig. 1 will operate at around a temperature of 2600 K. for a cathode current density of 3 amperes/cm.2. This temperature of operation will permit a much easier dissipation of the excess heat energy by the cathode structure.

At this high temperature of cathode operation, the life of the electrode depends upon the rate of evaporation of the tantalum cylinder I0 and the tungsten heater rod I2. It may be shown that the maximum life for the structure will resuit when both the center rod I2 and the electron emitting cylinder I have equal maximum lives. Tungsten may be run at about 200 K. above tantalum for an equal rate of evaporation. Therefore, to give the same operational life to both, the tungsten heater rod I2 is run at a higher temperature than the tantalum cylinder I0. Furthermore, it is advantageous t0 use tantalum as the emitting surface and tungsten as the heater rod, since to radiate any power to the outer cylinder Il the heater rod I2 must run at a higher temperature.

In operation, the heater current will flow through the tantalum cylinder I0 and back through the tungsten rod I2, and conversely. Both the cylinder IG and the rod I2 are made of materials having substantial electrical resistance, so that the cylinder It is heated directly by the heater current passing through it and also by radiation of heat to it from the rod I2, which is also heated by the heater current. Cathodes heretofore used relied on either (1) direct heating of the emitting element by passage 0f an electric current therethrough, or (2) heating of the emitting element by radiation from a separate heater element adjacent to the emitting element. My invention provides a cathode strucsaasaors ture which combines theseitwo :kinds of :heating of 1 the emittingelement. .Bychoosing the Wall thickness of theitantalum cylinder IU and the diameter of the tungstenfrod I2, in accordance with the conditions for maximum life, the 12R loss in each :will besuch that the temperature of the rod will .be higher during normal operation than that of the cylinder |10.

In Fig. 2, Vthere is shown somewhatzthe same type `of cathode structure -as is disclosed in Fig. 1. The tube in this figureislalso of the magnetron type. An anode ring `42.1with anode ns A4| forming cavity `resonators is spaced radially from a cathode emitting 'surface 43. -A magneticeld is established, .during tube operation, with the lines .of force of the field parallel to `.the axis of the anode ring 42. This magnetic held is established by -a permanent magnet 5I with pole pieces 5 andl respectively lpositioned on opposite `sides of the interelectrodespace .between the anode 42 and cathode emitting surfaceJiB. The pole pieces `5t and 52 are made, preferably, of steel. Pole piece Sii is set into a cylindrical chamber of the magnet 5l. Pole piece 52 is of a tubular construction Which permits the passage of the. cathode structure therethrough. This tubular piece 52 is mounted in the l.annular chamber 53 of the permanent magnet 5i. Anode ring 42 is sealed directly to pole piece 52. by a vmetal sealing ring '45. I'Iheother `side ofthe tube is closed by a copper plate IIBsealed `to the anode ring by a metal seaiing ring 4:4. Insulating strip 56, preferably of mica, separates the pole piece 5l] from the closure plate 48. Also, insulating material 54, preferably of mica, separates the pole piece 52 from the magnet 5i. Cathode structure 40 is mounted on a supporting cylinder-64 `.Within the hollow chamber of. the tube through pole piece 52. The cathode supporting cylinder 64 is xed to the pole piece 52 by a `metal ring 60 welded, lbra-Zed or hermetically sealed by Aother means to the pole pieceZ and by a glass cylinder S2 sealed both to the ring-S0 and to the outer surface of the supporting cylinder VE4. Cylinder 64 is preferably of a metal havingv a coeiiicient of expansion similar to glass such as Kovan The cathode member 4d which is similar to that of Fig. 1 comprises essentially of the emitting cylinder 43 and the heater rod S8 rigidly fixed to the emitting cylinder 43 at one end. The other end of heater rod ii is supported by rod GS which in turn is held axially spaced from the support cylinder 6d by an insulating glass seal 22. The glass seal 22 also closes one end of the supporting cylinder 654. Leads 3u and 32 are respectively connected to the support rod 6E and the supporting cylinder 6ft. Heating current for the cathode passes from the leads 3d and 32 to the heater elements 43 and te of the cathode by the conductors 64 and The glass seal at the end of cylinder 64 permits the interior of the tube structure to be evacuated and sealed off to provide a high vacuum tube. The operation of this tube of Fig. 2 is similar to that of tube disclosed in Fig. 1.

With the disclosed designs of Figs. 1 and 2 I have obtained a cathode structure which has several distinct advantages. I provide a cathode with a single end mounting Which permits all of the leads of the electrode to enter the tube at the same end. The cathode is so designed as to dissipate large amounts of back bombardment power. I have designed a cathode which also provides high electron emission densities of the order of amperes per square centimeter. The design of Fig. 1 in particular provides ease of fabrication andfas'sembl-y .and nally .the rugged construc- ,tion-of `the cathode structure givesthe electrode .a deiinite :and -consistent life.

Whilecertain speciiic embodiments have been illustrated `and described, itvfwill be ,understood that various changes .andmodiilcations may be made therein without departing from the spirit .and scope -of the invention.

'Whatl claim as new is:

l1. Acathode for :a magnetron comprising a cathode cylinder having an electron emitting surface for providing a source `of electrons, means 4for electrically heating said emitting surface, said means including said cylinder and a i heater rod within said cylinder, Vmeans coaxially and coextensively mounting said heater rod within and in spaced relationship With said cylinder, saidmounting means electrically connectingadjacent .ends-of saidheater rod and said cylinder .at one end-thereof, conductors respectively connected to the other ends of said rod and -said heater cylinder for supplying a heater current thereto, said conductors including a supporting cylinder tightly iitted around said cylinder and a center support rod insulatingly mounted at one end within and extending axially oi' said supporting cylinder, said heater rod being rixed to the other end of said supporting rod, each of said cathode cylinders vand said rod having substantial electrical resistance, the cross sectional area and resistivity of said rod and cylinder providing a higher temperature for the 4rod than for vthe cylinder during operation of said cathode.

2. A cathode kfor Ya magnetron comprising an electron emitting tantalum cylinder, means for electrically heating said cylinder, said means including `said cylinder and a tungsten heater rod, means coaxially and coextensively mounting said rod within and in spaced relationship with said cylinder, `said mounting means electrically connecting adjacent ends of said rod and said cylinder=at oneend thereof, conductors respectively connected to the other ends of said rod and said :cylinder "for 'supplying a heating current thereto, .said conductors including a supporting cylinder tightly tted around said tantalum cylinder and a center support rod insulatingly mounted at one end within said supporting cylinder and extending axially thereof, said center support rod having a hole drilled axially in the other end thereof, said heater rod being fixed within each of said holes, said tantalum cylinder and said heater rod having substantial electrical resistance whereby both said tantalum cylinder and said rod are heated substantially by the current supplied.

3. A cathode for a magnetron comprising a tantalum cylinder, means for electrically heating said cylinder, said means including said cylinder and a tungsten rod within said cylinder, said rod extending the whole length of said cylinder in spaced relationship thereto, means mounting said cylinder at one end to said rod, said mounting means providing an electrical connection between said cylinder and said rod, and conductors respectively connected to the other end of said cylinder and to the corresponding end of said rod for supplying a heating current thereto, each of said cylinders and said rod constructed with substantial electrical resistance whereby both said cylinder and said rod are raised to a high temperature by said heating current, the outer surface of said cylinder providing at said high temperature a source of electron discharge.

4. A cathode comprising a cylinder, having an outer surface adapted for electron emission, means for heating said emission surface, said means including said cylinder and a heater rod mounted within and coaxial with said cylinder in spaced relation thereto, said rod and cylinder being of substantial electrical resistivity metal, said cathode and cylinder registering at one end, means electrically connecting said registering ends together, a pair of conducting members comprising a second elongated cylinder of larger diameter than said rst cylinder and engaging at one end the other end of said metal cylinder and a second rod insulatingly supported at one end in said supporting cylindrical structure and coaxial with said heater rod, said second rod having a recess for receiving the other end of said heater rod, leads connected respectively to each of said conductors for connecting said cathode to a source of electrical energy, whereby said rod and cylinder of substantial resistivity metal are both heated by an electric current for electron emission from said cylinder surface.

5. A cathode electrode comprising a metal tube having an electron emissive outer surface, electrical resistance means for heating said surface tc thermionic emission, said means including said metal tube and a heater rod coaxially mounted within said metal tube and electrically connected in series to said metal tube, the electrical resistance cf said metal tube and said heater being approximately equal, a pair of coaxial leads, one of said coaxial leads connected to each of said metal tubes and said heater rod.

6. A cathode electrode comprising a tantalum y tube having an electron emissive outer surface between the ends thereof, electrical resistance means for heating said surface to thermionic emission, said means including said tantalum tube and a tungsten heater rod coaxially mounted within said tantalum tube, electrically conductive means connecting one end of said rod in series to said tantalum tube at one end thereof, the electrical resistance of said tantalum tube and said tungsten rod being approximately equal, a pair of coaxial leads, one of said coaxial leads connected to the other end of said heater rod, the other of said coaxial leads connected to the other end of said tantalum tube.

7. A cathode comprising a first elongated electrical conductor having an electron emissive surface, a second elongated electrical conductor mounted closely adjacent to said first conductor, low resistance means electrically connecting said conductors together at one end, and terminal means connected to said two conductors at the other end for supplying an electric heating current thereto, both of said conductors having substantial electrical resistance, the cross sectional area and resistivity of said tWo conductors providing a higher temperature for the second conductor than for the first conductor during operation of said cathode, whereby both of said conductors will be heated directly by said heating current and said rst conductor will be additionally heated by radiation of heat to it from said second conductor.

8. A cathode according to claim 7, wherein the electrical resistances of said two conductors are substantially equal.

9. A cathode according to claim 7, wherein the electrical resistance of said second conductor is higher than that of said iirst conductor.

10. A cathode according to claim 7, wherein said rst conductor comprises an imperforate tantalum tube, and said second conductor comprises a tungsten rod coaxially mounted Within said tantalum tube.

ROBERT L. JEPSEN.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,881,644 Jones Oct. 11, 1932 2,107,945 Hull et al Feb. 8, 1938 2,406,276 White Aug. 20, 1946 2,406,277 Bondley Aug. 20, 1946 2,411,601 Spencer Nov. 26, 1946 2,416,899 Brown Mar, 4, 1947 

